(a) Field of the Invention
The present invention relates to a light emitting display device and a driving method thereof. More specifically, the present invention relates to a light emitting display device using organic electroluminescence (EL) and a driving method thereof.
(b) Description of the Related Art
In general, an organic EL display electrically excites a phosphorous organic compound to emit light, and it voltage- or current-drives N×M organic emitting cells to display images. As shown in FIG. 1, the organic emitting cell includes an anode (an ITO anode or an indium tin oxide anode), an organic thin film, and a cathode layer (metal). The organic thin film has a multi-layer structure including an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL) for maintaining balance between electrons and holes and improving emitting efficiencies. Further, the organic thin film includes an electron injecting layer (EIL) and a hole injecting layer (HIL).
Methods for driving the organic emission cells are classified as a passive matrix method, and an active matrix method using thin film transistors (TFTs). The passive matrix method provides anodes and cathodes that cross (or cross over) each other, and selects a line to drive the organic emission cells. The active matrix method provides TFTs that access respective ITO pixel electrodes, and drives a line according to a voltage maintained by a capacitance of a capacitor accessed to a gate of a TFT. Further, depending on formats of signals applied to the capacitor for establishing the voltage, the active matrix method can be categorized as a voltage programming method and a current programming method.
The pixel circuit of the conventional voltage programming method has difficulties in obtaining high gray scales because of deviations of the threshold voltage (VTH) and the carrier mobility, the deviations being caused by non-uniformity of a manufacturing process. For example, in order to represent 8-bit (i.e., 256) gray scales in the case of driving thin film transistors by a voltage of 3V (volts), it is required to apply the voltage to the gate of the thin film transistor with an interval less than a voltage of 12 mV (=3V/256), and if the deviation of the threshold voltage of the thin film transistor caused by the non-uniformity of the manufacturing process is 100 mV, it is difficult to represent high gray scales.
The pixel circuit of the current programming method achieves uniform display characteristics when the driving transistor in each pixel has non-uniform voltage-current characteristics, providing that a current source for supplying the current to the pixel circuit is uniform throughout the whole panel.
However, the pixel circuit of the current programming method produces a long data programming time because of a parasitic capacitance component provided on the data line. In particular, the time (the data programming time) for programming the data on the current pixel line is influenced by a voltage state of the data line according to the data of a previous pixel line, and in particular, the data programming time is further lengthened when the data line is charged with a voltage which has a large difference from the target voltage (the voltage corresponding to the current data). This phenomenon becomes greater as the gray level becomes lower (near black). FIG. 1 shows a graph on variations of data programming times versus gray levels to be written in the conventional light emitting display device. The time t1 to t7 in FIG. 1 represents the data programming times, and the gray lines (e.g., gray 00 through gray 63) on the right of the graph indicate gray levels of the data programmed to the pixel circuit coupled to the previous pixel line.
For example, when the gray level of the data programmed to the pixel circuit coupled to the previous pixel line is “8” and the gray level of the data to be programmed to the pixel circuit coupled to the current pixel line is 8 (i.e., a point where a curve meets the horizontal axis), the time needed for data programming is almost “0” since there is no difference between the voltage state of the data line and the target voltage.
By contrast, the time needed for data programming increases as the difference between the voltage state of the data line and the target voltage increases because the gray level of the data to be currently programmed becomes farther away from the gray level of 8.
Also, the time needed for data programming is inversely proportional to the magnitude of the data current for driving the data line. As such, when the gray level is to be lowered, the data current for driving the data line is reduced, and hence, the data programming time is increased. That is, as can be derived from FIG. 1, when the gray level is lowered (e.g., to near the black level), the data voltage is changed to have a large voltage range with a low driving current, and the data programming time is increased.